In the EFG process tubular crystalline bodies, e.g., hollow bodies with round or polygonal cross-sections, are grown on a seed from a liquid film of a selected feed material which is transported by capillary action from a melt contained in a crucible through one or more capillaries in a die to the top end surface of the die. EFG dies typically comprise a top end surface, inner and outer side surfaces which intersect the top end surface, and at least one capillary, i.e., a passageway of capillary proportions, having an upper end which terminates at the top end surface of the die and a lower end which communicates with the melt in the crucible. The shape of the crystalline body is determined by the external or edge configuration of the top end surface of the die. Polygonally-shaped hollow bodies of silicon, e.g., "nonagons" or "octagons", grown by EFG process are subdivided at their corners into a plurality of flat substrates that are used to form photovoltaic solar cells. Early designs of EFG dies for growing silicon octagons are exemplified by U.S. Pat. No. 4,230,674 issued to A. S. Taylor et al., and U.S. Pat. No. 4,647,437 issued to R. W. Stormont et al.
Use of the EFG process is complicated by the fact that variations in temperature tend to exist around the circumference of an EFG die when the die is used to grow crystalline bodies. Variations in thermal symmetry around the circumference of the die can cause local changes in thickness of the growing crystalline body. Such variations in thickness may be severe enough to render the crystalline body more susceptible to fracture, thereby reducing the number of solar cells which can be produced from the hollow polygonal crystalline body, inasmuch as the thinner substrates cut from the crystalline body tend to be prone to breakage. Significant variations in temperature around the circumference of the die also make it difficult to sustain growth, resulting in rupturing of the liquid menisci that extend between the die and the growing crystal body. When the menisci are ruptured, i.e., when the liquid/solid growth interface is terminated, liquid silicon tends to splash or overflow onto inner and/or outer surfaces of the die, resulting in formation of local deposits of silicon carbide (SiC) on the die surfaces that adversely affected the emissivity and thermal conductivity of the die, causing the temperature distribution around the die to became even less uniform. Eventually the deposits become severe enough to render the die unsuitable for further use. Premature loss of a die increases the costs of producing hollow bodies by the EFG process.
The wet tip die design disclosed by U.S. Pat. No. 5,037,622, issued to Harvey et al., improved upon prior die designs by reducing variations in thermal symmetry around the circumference of the die, thereby improving the quality of the grown bodies and reducing the rate of occurrence of rupturing of the menisci. However, even with dies embodying the design of Harvey et al there is a tendency for molten silicon to solidify near the center hub region in the hot zone during crystal growth. The solidified silicon grows outward of the center hub region and forms a "mushroom-shaped" solid piece that ultimately is large enough to block replenishment of the melt. Such solidification near the center hub region also affects the uniformity of the growing crystalline body and disrupts growth. Also fluctuations in temperature can result in a portion of the mushroom-shaped piece breaking off and falling into the melt, causing the crucible to overfill and flood the die. Moreover, cool-down of the apparatus at the end of a growth run often is accompanied by fracture of the susceptor on which the crucible is supported, necessitating its replacement. Replacing a fractured susceptor is costly, particularly with respect to lost production.